1. Field of the Invention
The present invention relates to a novel oligosaccharide lipid which has a definite structure and is useful for stabilizing a phospholipid vesicle (hereinafter simply referred to as a "vesicle"). In the present invention, the oligosaccharide includes disaccharides.
The present invention also relates to a vesicle stabilizer which comprises an oligosaccharide lipid having a definite structure, and is useful for stabilizing a vesicle.
The vesicle stabilizer of the present invention modifies the vesicle, thereby inhibiting aggregation of the vesicles, stabilizing the dispersion of the vesicles against ions and proteins, or extending the half-life of the vesicles in blood stream.
The vesicle which has been modified with the stabilizer of the present invention is useful for drug-delivery systems and controlled drug-releasing systems in which physiologically active substances such as adriamycin and prostaglandin are enclosed in the internal aqueous phase of the vesicle. The modified vesicle which encloses purified hemoglobin is useful as an artificial red blood cell.
2. Description of the Related Art
Vesicles are normally unstable to come to aggregate or fuse together or to cause leakage of encapsulated molecules. A number of reports have been presented to stabilize the vesicles as mentioned below.
Vesicles are stabilized by polymerizing assemblies of phospholipids having an unsaturated groups (for example, see a review: H. Ringsdof, et al.: Angew. Chem., Int. Engl., 27, 113 (1988)). However, the polymerization of the membrane-constituting substance impairs the inherent characteristics such as gel-liquid crystal phase transition, phase separation, and membrane fluidity, causing loss of the functions based on these characteristics.
The vesicle structure of the phospholipid assemblies is stabilized by coating of its surface with carboxymethylchitin (H. Izawa, et al.: Biochim. Biophys. Acta, 855, 243 (1986)), and coating with polysaccharide compounds such as dextran and pullulan having a hydrophobic group such as cholesterol (J. Sunamoto, et al.: J. Biochem., 88, 1219 (1980)). Such a method is liable disadvantageously to induce aggregation of vesicles although the membrane fluidity of the vesicle is not greatly affected.
Vesicles are stabilized to aggregation by introducing of an amphiphilic molecule which is formed by bonding polyethylene glycol to a hydrophilic portion of cholesterol or phosphatidyl ethanolamine, thereby the half-life of the vesicles in blood being extended (T. M. Allen, et al.: Biochim, Biophys. Acta, 1066, 29 (1991).
A compound which is formed by introducing a hydrophobic moiety to a polyethylene glycol base is disclosed as a protein adsorption inhibitor to adsorb proteins to phospholipid vesicles and a vesicle aggregation inhibitor (Japanese Patent Application Laid-Open No. Hei-2-149512). However, the effect is not sufficient yet.
A glycosaminoglycan bonded to phospholipid exhibits cell adhesion inhibiting effect, and is useful as a cancer metastasis inhibitor (Japanese Patent Application Laid-Open No. Hei-4-80202).
A cholesterol derivative having .beta.-aminogalactose-moiety is employed for controlling aggregation of vesicles (P. S. Wu, et al.: Proc. Natl. Acad. Sci., USA, 78, 6211 (1981)). The effect of the derivative is not so high as that expected, and is suggested to depend largely upon the polymerization degree of the saccharide moiety.
A hyaluronic acid derivative is disclosed which stabilizes a vesicle dispersion system (Japanese Patent Application Laid-Open No. Hei 3-47801). The hyaluronic acid derivative does not exhibit sufficient stabilizing effect owing to the non-selective introduction of the acyl group by ester linkage, the effect depending greatly on the introduction sites and the number of the introduced acyl groups. Furthermore, the derivative frequently exhibits impractically high viscosity.
The inventors of the present invention disclosed that introduction of an oligosaccharide-fatty acid ester developed by the inventors into the bimolecular layer of a phospholipid vesicle is highly effective in inhibition of aggregation and fusion of vesicles, namely stabilization of the dispersing state of vesicles (Japanese Patent Application Laid-Open No. Hei 1-294701). Oligosaccharide ester derivatives having a longer saccharide chain give higher effect, and the derivatives based on disaccharides do not give sufficient effect on inhibition of aggregation of vesicles.
The invention disclosed in the above Japanese Patent Application Laid-Open No. Hei 1-294701 improves significantly the stability of vesicles. However, since the oligosaccharide has many reactive hydroxyl groups, the long-chain fatty acid group is introduced non-selectively to the oligosaccharide in a variety of numbers and a variety of sites in the oligosaccharide molecules. Consequently, isolation and purification of the product are complicated, and the yield is low. Moreover, the effect of stabilizing the vesicle dispersion and the fixation of the oligosaccharide-fatty acid ester onto the membrane surface of the vesicle depend disadvantageously on the kind of the isomers of the oligosaccharide, and the correlation of the stabilizing effect to the structure of the isomers is not clear.
The present invention intends to solve the above problems.